The present invention relates to the field of laser printers, laser copiers, photocopiers, laser fax machines and other image printing devices which use light to write an image to the electric charges on a printing drum. More particularly, the present invention relates to the field of minimizing toner explosions in such image printing devices, where such toner explosions are caused by the vaporization of moisture in the paper when the toner is fixed to the paper by exposure to a heating element.
Photocopiers operate on the following principles. An original document is laid on a glass window and illuminated from below by a powerful light. Typically, the light is scanned along the length of the original document. An optical system directs the light reflected from the document, i.e., an image of the document, to a printing drum. The printing drum is a cylindrical, metal drum which is given a negative charge at the beginning of the copying cycle. The drum is rotated as it is illuminated with the image of the document so that the image of the document is evenly and proportionally illuminated along the circumference of the drum without overlapping.
Where the image on the original document was bright, a relatively large amount of light will be reflected and directed to the printing drum. Conversely, where the image on the original document was dark, relatively little light is reflected to the printing drum.
The negative charge on the printing drum is dissipated in proportion to the amount of light falling thereon. Consequently, the image from the original document is effectively transferred to the charges on the printing drum, there remaining, for example, a substantial negative charge at darker portions of the image and little or no negative charge at the brighter portions of the image.
Positively charged toner is then applied to the printing drum. As positive and negative charges attract, the positively charged toner adheres to the printing drum in the pattern defined by the negative charge remaining on the drum, i.e., the dark or darker portions of the image. The stronger the negative charge on the drum, the more toner will adhere. The toner on the drum is then transferred to a piece of paper, thereby printing the image of the original document on the paper. The toner is then fixed or sealed to the paper by a heating element. Copying is then complete.
A laser printer works on the same general principles. However, instead of using an optical image obtained by illuminating an original, the laser printer receives a bit stream composed of digital data defining the light and dark portions of the image using a gray or color scale. This bit stream is used to modulate the light from a laser that is scanned over the printing drum.
In further contrast to a photocopier, where the bit stream defines a dark portion of the image, the laser will strongly illuminate the printing drum to dissipate the negative charge. Where the bit stream defines a light portion of the image, the laser may illuminate the drum weakly or not at all.
Negatively charged toner, rather than positively charged toner, is then applied to the printing drum. The negatively charged toner is repelled by the areas of the drum on which a negative charge remains (the light portions of the image) and adheres to the pattern of areas where the negative charge has been depleted from the drum by the laser in accordance with the image bit stream. The toner is then transferred to paper and heat sealed by, for example, a fuser nip.
A problem known as toner explosion occurs in all such laser and photocopier imaging systems. Toner explosion occurs when the toner, which has been transferred to paper, is sealed to the paper using the heat of a heating element. If there is moisture in the paper, as is common, that moisture will tend to suddenly vaporize when exposed to the heat that is applied to seal the toner. The sudden vaporization of the moisture in the paper may spread the pile of unsealed toner. Consequently, at that portion of the image, the toner is fixed in the resulting explosion pattern and degrades the image being printed.
Toner explosion problems are particularly frequent and noticeable when the printer is printing a solid horizontal line or edge and the moisture has no escape route without disturbing a toner pile. Typically, the explosion spreads toner below the line or edge, in a direction away from the direction in which the paper is being fed through the printing device to the heating element. The spread toner is then sealed to the paper in the unintended explosion pattern rather than as part of the horizontal line or other image element defined by a horizontal edge. These explosions are all the more noticeable against the straight horizontal line or edge from which they extend.
Consequently, in the field of laser imaging devices, there is a need in the art for a system and method of printing horizontal lines and edges, in particular, without causing toner explosions when the toner is heat-sealed to the paper or other print medium.
The present invention is directed to a system and method of printing horizontal lines and edges, in a laser printing device or digital copier, while reducing the distortion caused by toner explosions when the toner is heat-sealed to the paper or other print medium.
The present invention may be implemented in an image printing device in which a laser or other light source is modulated in accordance with an image bit stream in order to represent an image in charges on a printing drum which then transfers toner, directly or indirectly, to a print medium in accordance with the resulting charge pattern on the printing drum. In such an image printing device, the present invention preferably includes an input for receiving the image bit stream; and a processor for processing the image bit stream. The processor identifies segments of the image bit stream representing substantially solid horizontal lines or edges which are susceptible to distortion by toner explosions. The processor then modifies the segments of the image bit stream representing a substantially solid horizontal line or edge so as to decrease the amount of toner and toner pile heights that result at those portions of the printed image. This minimizes distortion caused by toner explosions. Specifically, the processor modifies the image bit stream representing a substantially solid horizontal line or edge such that the modified bit stream then represents an outlined area containing a fill pattern that replaces the substantially solid horizontal line or edge.
As will be appreciated by those skilled in the art, the modification of the image bit stream could also be accomplished by a processor in a host computer or printer client device before the image bit stream is transmitted to the image printing device.
Image printing devices in which the invention may be implemented will typically also include a light source driver and a light source (e.g. a laser). The driver drives the laser in accordance with the image bit stream as modified. Preferably, the modified bit stream is modified so as to cause the laser to make a center justified mark of a particular width in each pixel of the fill pattern. More preferably, the modified bit stream is modified so as to cause the laser to make marks in pixels in a single column of pixels with different justifications. Specifically, the modified bit stream preferably causes the laser to make center-justified marks in a first row of pixels of the fill pattern and split-justified marks in a second row of pixels of the fill pattern.
The present invention also encompasses the methods of making and operating the system described above. Specifically, the present invention encompasses the method of minimizing distortions due to toner explosions in images produced by an image printing device in which a laser is modulated in accordance with an image bit stream in order to represent an image in charges on a printing drum which then transfers toner to a print medium accordingly to complete printing, by monitoring the image bit stream to identify segments of the image bit stream representing substantially solid horizontal lines or edges which are susceptible to distortion by toner explosions; and modifying the segments of the image bit stream representing a substantially solid horizontal line or edge which is susceptible to distortion by toner explosions so as to decrease an amount of toner and toner pile heights resulting in those portions of a printed image corresponding to the identified segments of the image bit stream representing a substantially solid horizontal line or edge so as to minimize distortion caused by toner explosions.
The present invention also encompasses the software or computer-readable instructions that cause the printing device to implement the present invention in which the fill pattern replaces horizontal lines and edges to minimize distortion due to toner explosions.